How do hydraulic load and intermittent aeration affect pollutants removal and greenhouse gases emission in wastewater ecological soil infiltration systems?

Highlights

• Hydraulic load and aeration affected pollutants removal, CO₂ and N₂O emission.

• Hydraulic load of 0.2 m/d was suggested for intermittent aeration WESISs.

• Aeration enhanced the abundances of bacterial 16S rRNA and nitrogen removal genes.

Abstract

This study investigated the effects of hydraulic load and intermittent aeration on matrix oxygen concentration, COD removal, NH₄⁺-N removal, TN removal, TP removal, greenhouse gases (GHGs) emission and functional gene abundances in wastewater ecological soil infiltration systems (WESISs). Intermittent aeration created aerobic conditions above 40 cm depth and did not change anaerobic conditions below 70 cm depth, which enhanced the abundances of bacteria and functional genes related to organic matter and nitrogen removal and improved COD, NH₄⁺-N, TN and TP removal compared with non-aeration. The pollutants removal efficiencies, CO₂ conversion efficiency decreased and N₂O conversion efficiency increased with hydraulic load increasing in the non-aeration WESIS. COD, NH₄⁺-N, TP removal efficiencies and CO₂ conversion efficiency decreased in the intermittent aeration WESIS when hydraulic load increased. As hydraulic load increased from 0.05 to 0.4 m/d, TN removal efficiency first increased and then decreased in the intermittent aeration WESIS. However, N₂O conversion efficiency first decreased and then increased. Intermittent aeration WESIS achieved high removal efficiencies of 89.4%, 90.8%, 87.2%, 92.9% for COD, NH₄⁺-N, TN, TP, high conversion efficiency of 92.8% for CO₂ and low conversion efficiency of 0.19% for N₂O at hydraulic load of 0.2 m/d. Intermittent aeration and hydraulic load had hardly any effects on CH₄ emission.

Introduction

Wastewater treatment systems contain centralized treatment technologies and decentralized treatment technologies. Activated sludge and biofilm process are traditional centralized wastewater treatment technologies applied in cities which need high investments, operation and maintenance costs (Lam et al., 2015). Decentralized wastewater treatment technologies such as land treatments overcome these shortcomings. In addition, it could not bring biological sludge by-product (Kamble et al., 2017; Tietz et al., 2008; Torrens et al., 2009; Wang et al., 2010a).

Wastewater ecological soil infiltration system (WESIS) is one of land wastewater treatments, which feeds wastewater into a certain depth of constructed or natural soil (Wang et al., 2010b). Organic matter, nitrogen, phosphorus and pathogens are purified by soil adsorption, interception and filtration, chemical reaction, microbial degradation and plant absorption as wastewater infiltrated by capillary force and gravity force (Liang et al., 2019; Yang et al., 2016). It has been reported that the WESIS is an effective ecological technology for organics and phosphorous removal (Fan et al., 2013; Kong et al., 2002; Li et al., 2012). However, nitrogen removal is unsatisfactory. Nitrification and denitrification by microbes are recognized as the primary nitrogen removal mechanisms for WESISs, which need aerobic conditions for nitrification and anaerobic conditions for denitrification, respectively (Li et al., 2011; Wu et al., 2015). It is difficult to be implemented simultaneously in conventional WESISs because oxygen from air diffused to the soil was limited and the dominant environment in the system was anoxic or anaerobic (Fan et al., 2013; Pan et al., 2015). Many researchers have investigated the methods for improving nitrogen removal in WESISs. Sun et al. (2019) and Liang et al. (2019) amended the original soil with biochar. Li et al. (2011) and Wang et al. (2010b) adopted influent shunt distributing. Lloréns et al. (2011) applied vertical distribution or horizontal distribution of influent to improve nitrogen removal. However, TN removal efficiencies increased about 10% through above strategies. Intermittent aeration has been proved to be a feasible strategy for WESISs to enhance pollutants removal (Jiang et al., 2017; Pan et al., 2015; Yang et al., 2016). Influent hydraulic load is an important factor for WESISs which would affect purification effect and long-time running (Li et al., 2011; Zhang et al., 2005). Li et al. (2011) and Zhang et al. (2005) reported that COD, NH₄⁺-N and TN removal efficiencies decreased with the increase of hydraulic load in the non-aeration WESISs. However, the effect of hydraulic load on pollutants removal of intermittent aeration WESISs is very little.

CO₂, CH₄ and N₂O are three main greenhouse gases (GHGs) which emitted from WESISs during wastewater treatment (Itokawa et al., 1993; Kong et al., 2002). Generally, CO₂ occurs in the aerobic decomposition of organic matter activated by aerobic bacteria. CH₄ emission is ascribed to anaerobic decomposition of organic compounds by methanogen. N₂O produces in the aerobic or anoxic/anaerobic environment by nitrifying and denitrifying bacteria (Li et al., 2018). Recently, more attentions have been paid on GHGs emission of WESISs (Li et al., 2017, Li et al., 2018, Li et al., 2019; Kong et al., 2002, Kong et al., 2016; Zheng et al., 2018a, Zheng et al., 2018b). Li et al. (2019) reported that the anaerobic layer of WESIS was the main N₂O emission source. Kong et al., 2002, Kong et al., 2016 found that soil ORP and influent C/N had influences on N₂O and CH₄ emission in the WESIS. Zheng et al., 2018a, Zheng et al., 2018b concluded influent C/N ratio and surface organic load affected N₂O emission and pollutants removal, and as well intermittent aeration is a viable way to decrease N₂O emission for WESISs. However, few studies on three GHGs emission of WESISs have been implemented.

This study investigated two lab-scale WESISs operated with/without intermittent aeration under different influent hydraulic loads. The purposes were to: (1) evaluate the effects of influent hydraulic load and intermittent aeration on pollutants removal and GHGs emission; (2) analyze spatial distribution of bacteria and functional genes which participated in organic matter removal, nitrogen removal, CO₂, CH₄ and N₂O emission under different influent hydraulic loads with/without intermittent aeration. We conduct the following hypotheses.

• Intermittent aeration enhances pollutants removal and influent hydraulic load decreases pollutants removal of the WESISs.

• Intermittent aeration enhances CO₂ conversion efficiency and decreases N₂O conversion efficiency. Influent hydraulic load have different influences on CO₂, CH₄ and N₂O emission in intermittent aeration and non-aeration WESISs.

• Intermittent aeration and influent hydraulic load affect the abundances of bacteria and functional genes involved in organic matter removal, nitrogen removal, CO₂, CH₄ and N₂O emission.